1. Field of the Invention
This invention is directed to a system for determining a direction of incident electromagnetic signals. More particularly, this invention is directed to a system for determining the angle of arrival of a radio frequency signal, in one or two dimensions, utilizing a pair of simple wideband broadbeam spaced apart antennas in a receiver which includes an analog spectral separator. Further, this invention directs itself to analog spectral separation schemes utilizing narrow band filtering of radio frequency or intermediate frequency signals that are digitized utilizing analog to digital converters having a relatively low sampling rate. Still further, this invention is directed to a method for determining a direction of incident electromagnetic signals where received signals from a pair of antennas are processed by analog circuits to provide a plurality of signal samples that provide a frequency domain representation of the received signal from which phase difference information can be extracted and utilized to compute an angle of arrival for the received signal.
2. Prior Art
Many applications require the ability to determine the location of an emitter of electromagnetic signals, especially those associated with the military. Military aircraft, for example, have a need to sense the radio frequency environment for real-time defensive and offensive purposes. The direction from which an attack may be coming is critical information required by the pilot, which may be ascertained by identifying the direction from which a particular radar beam is being emitted. A number of methods for determining the angle of arrival for such radio frequency signals have been developed. One such method is the triangulation of coordinated receiver data from multiple widely displaced aircraft. For individual aircraft, several techniques have been developed, including an amplitude-difference approach, an interferometer approach, and a time difference of arrival approach. When simple broadbeam wideband antennas are installed, these approaches generally determine the angle in just one dimension. Most commonly, the antennas are separated horizontally, and often the measured angle of arrival is treated as being equal to the azimuth angle based on the assumption that the signal originated near the horizon, making the elevation angle approximately zero.
The amplitude-difference angle of arrival measurement approach is a commonly used approach for military fighter aircraft because it is relatively simple and inexpensive to implement. The amplitude-difference approach is based on the utilization of antenna gain variation as a function of angle of arrival. Such a system makes its angle of arrival determination by comparing the intercepted amplitude at each of the plurality of antennas disposed around the aircraft, and deducing the angle that must have caused the amplitude ratios therebetween. The problem with the amplitude-difference approach is that it is not very accurate because of calibration difficulties. These systems need to operate on a wideband basis because the radar that they are intended to intercept can be operating at any frequency across a wide range of possibilities. Although normally the processing is independent of carrier frequency, since the amplitude measurements are usually based on the signals (video) envelope (the carrier having been stripped off), calibration issues create an indirect frequency dependence. The antenna patterns vary considerably with frequency, and although calibration lookup tables are helpful, they are difficult to determine utilizing ground measurements and vary from individual aircraft to aircraft and may even vary based on the varying weapons configuration of each aircraft. Gain variations as a function of the unknown elevation angle also introduces significant errors.
The interferometer approach, while providing angle of arrival measurements that have high resolution and accuracy, are expensive and usually impractical to mount on a fighter aircraft. Such systems are usually limited to a larger intelligence gathering aircraft. The multiple antennas of the system are physically separated, and for long-baseline interferometers, the sensors are especially spaced far apart. Due to the physical separation of the antennas, the propagating wave""s carrier will generally have a unique phase angle for each individual antenna location. The receiver measures this carrier phase for the received pulse and passes the phase data to a central processing unit. The central processing unit takes the difference of the phase, multiplied by a constant, and uses trigonometry to solve for the angle of arrival. The constant utilized in the calculation depends on the distance between the sensors, the speed of propagation of the wavefront and the frequency of the carrier. For the interferometer approach, the angle of arrival of resolution is proportional to the antenna separation. However, because the carrier phase angles have a modulo 360xc2x0 characteristic, an antenna system with a long baseline, operating in the frequency bands of interest, will not provide a unique solution. For example, such a system that measure 10xc2x0 difference between the signals intercepted by the two antennas, would not know whether the true additional propagation delay corresponded to 10xc2x0, 370xc2x0, 730xc2x0 or any other combination of 10xc2x0 plus a multiple of 360xc2x0. Thus, to get a unique solution, the interferometer approach requires additional antennas at precise locations between the two furthest-apart antennas. The antenna locations are precisely selected so that, for any frequency in the band of interest, the set of phase differences provides a unique solution for the angle of arrival. A typical interferometer system will have from three to five antennas to cover the field of view, depending on the operating bandwidth and center frequency, the greater the bandwidth, the more antennas needed to resolve the ambiguities. Further, the multiple antennas are usually pointed in the same direction, and the overlapping beam widths are made as wide as possible, sometimes approaching 180xc2x0. The number of antennas required for the interferometer approach creates many difficulties for application on fighter aircraft. Whereas a quadrant amplitude-difference approach would use just four 90xc2x0 beamwidth antennas, the interferometer approach might need fifteen antennas (five 120xc2x0 beamwidth antennas for each of three fields of view). The number of antennas and cabling required therefor also adds to the complexity and difficulty in calibrating such a system.
The time difference of arrival approach determines the angle of arrival by measuring the time difference between when the RF wavefront strikes two antennas. The approach is generally implemented with a high-speed counter that measures the time between threshold crossings of the envelope of the signals intercepted by the two antennas. The envelopes, or video signals, are generated by wideband detectors which rectify the RF carrier voltage. The angle of arrival is calculated utilizing conventional mathematical and trigonometric processing. Since the time difference of arrival approach uses the envelope, stripping off the carrier from the signal, the approach has the advantage that the processing normally does not depend on the carrier frequency, unlike the interferometer approach. A fixed envelope-delay error can generally be calibrated out easily at any one carrier frequency, with the calibration result being good for the whole band. However, like the amplitude-difference approach, there is an indirect-frequency dependence via frequency-dependent gain differences in the antenna and other front-end components. While the time difference of arrival approach would have good resolution for acoustic or vibrational waves, the propagation speed of electromagnetic waves are so fast that it is difficult to count correspondingly fast to get adequate resolution. The antenna spacing limitations on military aircraft requires a resolution better than one nanosecond, which is difficult to obtain. Further, if the pulse""s leading edge has a significant slope, then gain variations between the two channels will result in the time between threshold crossings not being equal to the time difference of arrival, causing significant angle of arrival measurement error. Further, such fine-grain time measurement requires very wide receiver bandwidths, the wide bandwidth receiver then being subject to receipt of interfering signals and noise along with the signal of interest. The interference caused by the unwanted simultaneous receipt of signals is especially difficult to deal with. Additionally, for signal modulations other than pulse modulation, such as frequency modulation or phase modulation, the time difference of arrival implementation completely fails to make the required measurement. Even if a modern digital receiver approach is implemented to provide the programming flexibility to deal with frequency or phase modulated signals, and to include filtering to reduce the effects of interference, the characteristics of the analog to digital converters needed result in poor resolution. The sampling period of the converters needs to be much less than the expected time different of arrival. On the one hand, where high amplitude resolution analog to digital converters are used, such do not have a sufficiently high sampling rate and thus produce a time difference of arrival measurement that is too coarse. On the other hand, if sufficiently fast analog to digital converters are utilized, then such will have coarse amplitude resolution and make the system vulnerable to non-linear intermodulation-causing interference by strong simultaneous signals.
A variation of the time-difference-of-arrival approach is to use the programming flexibility of digital receivers to make the needed calculations using frequency-domain processing; the phase slope with respect to frequency being proportional to the time difference of arrival. However, it does not matter if the processing is in the time domain or the frequency domain; practical analog to digital converters cannot provide the needed information for the aforesaid reasons.
In order to overcome the problems of the conventional methods for measuring angle of arrival, the present invention provides a system and method for measuring the angle of arrival in two coordinates, such as azimuth and elevation, using just two horizontally-separated broadband widebeam antennas with overlapping fields of view, utilizing analog processing to spectrally separate the received signal and digital processing to make the needed calculations based on the spectral phase values. The angle of arrival measurement approach of the present invention measures pairs of phase values for two antenna inputs at two or more spectral locations within the signal bandwidth, subtracts to get the phase difference for each of the pairs of values, subtracts to get the phase difference values with frequency change, computes the phase-difference slope with respect to frequency, and uses mathematical and trigonometric processing to derive the angle of arrival value. The system therefore is able to operate with a minimum number of antenna sensors, can accommodate reasonable frequency-dependent tolerances of the antennas, utilizes narrow bandwidth receiver front-end components, is not sensitive to component tolerances, and determines the angle of arrival by frequency domain processing utilizing front-end analog filters whose outputs are digitized with relatively slow high resolution (high dynamic range) analog to digital converters. The second angular dimension is determined by comparing the computed time difference of arrival with the computed spectral magnitudes.
A system and method for determining a direction of incident electromagnetic signals is provided that includes at least a pair of spaced antennas having overlapping fields of view for respectively receiving a transmitted signal. The system further includes at least a pair of receivers respectively coupled to the pair of antennas. Each of the receivers includes a spectral separator having a plurality of output signals. Each of the output signals represents a signal value at one of a plurality of predetermined frequencies. A plurality of analog to digital converters are coupled to a corresponding one of the spectral separators for respectively providing a digital representation of each of the output signals. The system includes a digital processor coupled to an output of each of the plurality of analog to digital converters for calculating an angle of arrival of the transmitted signal from the digital representations of the output signals from the spectral separators.
From another aspect, a method for determining a direction of incident electromagnetic signals includes the steps of:
a. receiving a transmitted signal at each of at least two antennas to provide at least a first radio frequency signal and a second radio frequency signal;
b. demodulating and spectrally dividing the first radio frequency signal to provide a plurality of analog first output signals respectively corresponding to signal values at different ones of a plurality of predetermined spectral locations;
c. demodulating and spectrally dividing the second radio frequency signal to provide a plurality of analog second output signals respectively corresponding to signal values at the different ones of the plurality of predetermined spectral locations;
d. converting the plurality of analog first and second output signals to a plurality of first and second digital representations thereof; and,
e. calculating an angle of arrival of the transmitted signal from the first and second digital representations of the analog first and second output signals.
From yet another aspect, a method for determining a direction of incident electromagnetic signals is provided which includes the step of receiving a transmitted signal at each of at least two spaced apart antennas having overlapping fields of view to provide at least a first radio frequency signal and a second radio frequency signal. The method includes the step of coupling the first radio frequency signal to a first plurality of receivers. Each of the first plurality of receivers has a respective bandwidth less than a signal bandwidth of the transmitted signal and center frequency offset from the others of the first plurality of receivers to establish a plurality of analog first output signals. The method includes the step of coupling the second radio frequency signal to a second plurality of receivers. Each of the second plurality of receivers having a respective bandwidth less than a signal bandwidth of the transmitted signal and a center frequency offset from the others of the second plurality of receivers to establish a plurality of analog second output signals. The method includes the step of converting the plurality of analog first and second output signals to a plurality of first and second digital representations thereof. Still further, the method includes the step of calculating an angle of arrival of the transmitted signal from the first and second digital representations of the analog first and second output signals.